Electro-depositing stainless steel coatings on metal surfaces



United States PatentOfilice 3,374,156 Patented Mar. 19, 1968 This invention relates to the electro-deposition of stainless steel coatings on metal surfaces. It is particularly concerned with processes by which stainless'steel coatings having greatly improved corrosion resistance may be electro-deposited on various metals. The term stainless steel as used herein refers to ternary alloys containing from about 8 to about 27% chromium, from about 3 to about 33% nickel, and the balance iron.

The art of forming stainless steel coatings by electroplating is relatively new. The principal teaching in this field is U.S. Patent 3,093,556 and its foreign counterparts. Full details of processes for forming stainless steel coatings on metal surfaces are given in this patent and its teachings are incorporated herein by reference. The general processes taught by the patent include the use of aqueous electro-plating baths containing urea and soluble salts of the three metals found in the stainless steel. The patent also teaches that boric acid is a desirable component of such plating baths.

While the teachings ofthe above patent yield results which are quite acceptable for many purposes, it'has been found that the coatings formed by the process of the patent do-not' display all of the desirable properties of stain less steel coatings formed in'other ways. For example, the stainless steel coating formed in accordance with the patent is relatively thin and, for reasons which are not fully understood, does not exhibit the degree of corrosion resistance characteristic of thicker layers of stainless steel, such as those which may be rolled onto metal surfaces. As is well known, corrosion resistance is .oneof the many desirable properties of stainless steels and the failure of electro-deposited coatings of the patent to exhibit all of the characteristic good qualities of stainless steels generally has been a puzzling drawback to the full exploitation of the breakthrough represented by the teachings of U.S. Patent 3,093,556. Furthermore, chemical passivating treatments and heat treatments of the kind used in the plating art do not materially improve the corrosion resistance of electro-plated stainless steel coatings.

It has been found that by relatively simple modifications of the baths and thetechniques taught by the aforementioned patent, quality and corrosion resistance of electro-plated stainless steel coatings can be markedly improved. Thus the principal object of this invention is the provision of processes-for forming stainless steel coatings electrolytically whereby the coatings produced will be greatly improved in quality and corrosion resistance.

For background purposes, the general plating bath compositions and operating "conditions of Patent 3,093,556 will be summarized here because the present'invention finds its principal utility in the improvement of methods which fall within the general teachings of the patent.

' It should first benoted that the process yields stainless steel alloy coatings in which the chromium content is from about 8.2% to about 217.1%, the nickel from about 3.6% to about 33% and iron the balance of the coating. Within the above ranges, the iron content is always predominant-- 1y greater than either chromium or nickel and is at least 58.5%.

The plating bath should contain at least 0.2 mol/liter of each of the three ions to be deposited, although the upper concentration limit is not as vital and may be as high as saturation for each ion. Sulfate and chloride salts, including hydrated salts, may be used to supply the metal ions.

Urea in amounts between about 1.0 and 4.0 mols/liter is another component of the bath. In general, it has been found that in order to obtain plated coatings having compositions of the kind usually referred toas stainless steel,

the concentration of urea must beheld within the foregoing limits. I

' It is preferred that the pH of the plating bath be maintained between about 1.5 and 3.5. The pH can be adjusted by addition of free acid or base to the bath.

7 Aside from pH, the other plating bath operating variables which are of importance are current density and operating temperature. The current density may be varied between about 0.645 to 2.148 amps per square inch and variations in the current density will cause changes in the relative proportions of the three metals forming the coating. In general, an increase in current-density tends to cause an increase in the chromium concentration in the alloy and a decrease in the nickel. The operating temperature should be held between about 30" and C. Theop'erating-temperature, like the current density, alfects the relative proportions of the metals in the plated alloys. In.

general, lower temperatures tend to increase the propor-' tions of chromium at the expense of nickel. The time of treatment and the nature of the piece being coated werefound to, have ,but little eifect on the composition of the alloy whichis. deposited.

It hasbeen found also that boricacid is a desirable optional ingredient in .the plating baths since it appears to have the elfect of smoothing the coating and making it more homogeneous. The concentration of boric acid should be between about 0.2 and 0.6 mol/liter. Various other subsidiary ingredients can be included, such as pyridine, which acts as a brightening agent. Another subsidiary ingredient, not specifically mentioned in Patent 3,093,556 is citric acid. Its use in the present invention is optional, but if it is used, it tends to smooth out the coat-,

ing and to brighten it.

discussed below is incorporated as an additional ingredient. in coating baths falling within the general teachings out-.

The preferred and operative ranges of hydrazine will be described fully below in the discussion of the illustrative examples but these ranges can be summarized here.

It has been found that the amount of hydrazine used should be from about 0.3 gram/liter to about 3.0 grams/ liter, calculated as N H when the coating baths and their operating conditions in other respects follow the teachings of U.S. Patent 3,093,556 outlined above. The concentration limits just given are imposed by practical considerations. Thus, it hasbeen found that if the concentration of the hydrazine falls below about 0.3 m'ol/liter, the improvement in corrosion resistance obtained is so small. that itvis of no. .practical valuecwhen the concentration of hy-,1

drazineisincreased above the level of about 3.0 grams per liter, the results are not materially impaired but the degree of improyement obtained by such increases in the concentration of hydrazine is negligible and not worth the extra cost. I

Within the broad concentration range for hydrazine set forth above, it has been found that the corrosion resistance is optimized if the hydrazine content is at least about l 1.8 'g./l. When plating baths have been operated with the hydrazine concentration held between about 1.8 and 3.0

I havediscovered that if a hydrazine in amounts to be g./l., the coatings formed have resisted failure in the standard salt fog test for hundreds of hours.

For the purposes of this invention, any hydrazine which is soluble in the plating baths, and which does not cause the precipitation of the other components of the plating bath, is suitable. Thus, the term hydrazine as used herein is meant to denote a class of-compounds, rather than a single compound. Typical hydrazines which work in accordance with the invention are: hydrazine (N H methyl hydrazine, dimethyl hydrazine, phenyl hydrazine, hydrazine sulfate, and hydrazine chloride.

The base metals which can be coated in accordance with the processes of this invention include the group consisting of iron, nickel, zinc, and alloys of each wherein the particular metal constitutes the principal ingredient of the alloy. The improved corrosion resistance of coatings formed in accordance with the invention is obtained for coatings on all of these base metals, and the nature of the base metal has substantially no effect on the corrosion resistance properties.

It should be noted that the hydrazines are bases and will thus tend to raise somewhat the pH of plating baths in which they are included. However, it is still preferred that the pH be maintained between about 1.5 and 3.5, as taught by the aforementioned patent. Final adjustment of the pH is desirably accomplished by addition of acid or base to the bath.

The following illustrative examples demonstrate the practice of this invention and the marked improvement in corrosion resistance which results.

A stock solution corresponding to:

Grams/ liter NiSO .6H O 112 FeC1 .4H O H 30 25 Urea 180 Citric acid was prepared. In this example the ingredients were slowly dissolved in water at 90 C., allowed to stand at 90 C. for 1 hour, and then permitted to cool overnight befor use.

The stock solution was used to form plating baths for a series of tests in which stainless steel coatings were electro-plated on steel panels. Some of the panels were coated in a bath containing the stock solution, without modification, and others were plated in a bath containing stock solution to which hydrazine had been added. In this way, the effect of the use of hydrazine on the corrosion resistance properties of the coatings was isolated and observed.

Steel panels cleaned by solvent cleaning, followed by cathodic cleaning were electro-plated in baths under the conditions tabulated below:

Current Hydrazlne, Density g. [1.

a./in.

Time, Mins.

Panell No.:

resistance for plated, painted, or otherwise coated metal surfaces. It involves exposing the panels to be tested to a .finemist or fog of brine having a salt concentration of about 5%. The temperature within the test cabinet is maintained at about F. The time to failure of a panel being tested in the salt fog cabinet was determined by periodic visual inspection for evidence of pitting and corrosion. As soon as such evidence appeared, the panel was regarded as a failure. The duration of the test for the above panels was 1000 hours unless a given panel failed sooner. The panel ratings were as follows:

Failure Time Failure Time No Failure at Less than 840 hours 1,000 hours 1 hour Panel No.:

1 X 2 X 3 X 4 X 5. X 6. X

8 X 9.- X 10. X 11 X 12 X Another stock solution was prepared for a series of tests illustrating the effect of hydrazine on corrosion resistance and the effect of changes in the concentration of hydrazine on corrosion resistance. The stock solution had the following composition:

Grams/ liter KOr(SO -'1'ZH O 200 NiSO -6H O 112 Urea 1:80 Boric acid 25 Citric acid 15 Steel panels were cleaned in the manner previously described. These were plated in coating baths made from the stock solution under the conditions shown in the Table below.

The plated panels were iven the same post treatment as were the panels in the previous series of examples. The dried panels were then subjected to the ASTM salt fog cabinet test until failure. The failure times were as follows:

Panel No.: Salt spray failure-hours 1 4 From the results of this series of tests it can be seen that a discernible and material improvement in the corrosion resistance of the plating can be observed when the hydr-azine concentration is as low as 0.3 g./l. It can also be seen that a vast increase in corrosion resistance is attained if the hydrazine concentration is raised to 1.8

5 g./l. The behavior of those panels plated in baths containing 3 g./l. of hydrazine is also excellent.

We claim:

1. The method of electro-depositing a stainless steel alloy coating consisting of chromium, nickel and iron on a metal surface, said method comprising using the metal to be coated as a cathode in a plating bath consisting essent-ially of an aqueous solution of from 0.2 mol/liter to saturation of a salt of each of the metals in the alloy to be deposited, said salt being selected from the class cons-isting of sulfate salts and chloride salts of each of said metals, from 1 to 4 mols/liter of urea, at least 0.3 gram per liter of a soluble hydrazine, calculated as N H with the balance water; maintaining the temperature of the bath at from 30 C. to 80 C., and the pH at from 1.5 to 3.5, and passing an electric current of from 0.645 to 2.148 amps/in. therethrough between an anode and the said cathode.

2. The method according to claim 1 wherein the concentration of said hydrazine calculated as N H is between about 0.3 gram per liter and about 3.0 grams per liter.

3. The method according to claim 2 wherein said hydrazine is selected from the class consisting of hydrazine (N 'H methyl hydrazine, dimethyl hydrazine, phenyl hydrazine, hydrazine sulfate, and hydrazine chloride.

4. The method according to claim 1 wherein the concentration of said hydrazine calculated as N H is between about 1.8 grams per liter and about 3.0 grams per liter.

References Cited UNITED STATES PATENTS 3,093,556 6/1963' Machu et a1. 2044 3 HOWARD S. WILLIAMS, Primary Examiner. JOHN H. MA-CK, Examiner. G. KAPLAN, Assistant Examiner. 

1. THE METHOD OF ELECTRO-DEPOSITING A STAINLESS STEEL ALLOY COATING CONSISTING OF CHROMIUM, NICKEL AND IRON ON A METAL SURFACE, SAID METHOD COMPRISING USING THE METAL TO BE COATED AS A CATHODE IN A PLATING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF FROM 0.2 MOL/LITER TO SATURATION OF A SALT OF EACH OF THE METALS IN THE ALLOY TO BE DEPOSITED, SAID SALT BEING SELECTED FROM THE CLASS CONSISTING OF SULFATE SALTS AND CHLORIDE SALTS OF EACH OF SAID METALS, FROM 1 TO 4 MOLS/LITER OF UREA, AT LEAST 0.3 GRAM PER LITER OF A SOLUBLE HYDRAZINE, CALCULATED AS N2H4, WITH THE BALANCE WATER; MAINTAINING THE TEMPERATURE OF THE BATH AT FROM 30*C. TO 80*C., AND THE PH AT FROM 1.5 TO 3.5, AND PASSING AN ELECTRIC CURRENT OF FROM 0.645 TO 2.148 AMPS./IN.2 THERETHROUGH BETWEEN AN ANODE AND THE SAID CATHODE. 